Fittings are commonly used to connect metal tubes and pipes to each other for use in a variety of applications, such as in the aerospace industry, to convey fuel, hydraulic control fluids and the like in an aircraft or space vehicle. In these applications, it is critical that there be a secure connection between the fitting and the tubes in order to withstand vibration and other adverse conditions without failure.
Various fittings have been developed in the past to connect tubes to each other. In one type of fitting, a radial swaging force is applied to the fitting and the tube, which may be done externally around the fitting or internally within the tube. In either case, the radial swaging force is applied directly to the fitting and tube by the tool. In some instances, the inner surface of the fitting has a plurality of axially spaced annular grooves or teeth by which the material of the tube is deformed by the swaging tool to make the swaged connection. In other instances, a curved or irregular configuration on the outer surface of the fitting is transferred to the inner surface of the fitting upon swaging by deforming the fitting, which causes the tube to deflect and conform to the irregular configuration and thereby make the connection.
Another type of fitting comprises a cylindrical sleeve having a tapered outer surface and a cylindrical inner surface for receiving a tube. A deforming ring surrounds the sleeve and has a tapered inner surface which matches and engages with the tapered outer surface of the sleeve. Before swaging, the deforming ring is positioned outwardly with respect to the sleeve such that no radial force is applied by the deforming ring to the sleeve. During swaging, the deforming ring is moved axially in a forward direction over the sleeve such that the interaction of the tapered surfaces on the ring and the sleeve applies a radial force deforming the sleeve and the tube inwardly to make a swaged connection between them. These fittings shall be generally referred to as axially swaged fittings.
Carbon fiber reinforced plastic (“CFRP”) materials are increasingly being used in place of aluminum to form the skin panels and structural members of commercial airplanes. CFRP materials are advantageous compared to aluminum due to the higher strength-to-weight ratios provided by carbon composites. However, CFRP materials offer less EME protection to systems installed inside the structural members from lightning strikes than aluminum materials. This is attributable to the intrinsic resistance of the composite materials. As a result, the conductive systems within the fuel tank may be forced to carry more of the current induced by a lightning strike event. Native hydraulic fittings within a composite fuel tank may not always be capable of carrying the higher current from a lightning strike to a CFRP airplane without sparking. In an airplane such sparking is not permitted in the presence of flammable vapor as an ignition source, such as in the presence of fuel vapors inside a fuel tank. Therefore, the use of CFRP materials requires special considerations of the effects of lightning strikes and other electro-magnetic effects (EME). This is particularly important in the wing structure where metallic tubing for fuel transfer and hydraulic actuation pass through the fuel tank.
Current solutions include the use of isolators of short sections of tube, which can be used to interrupt the current flow while permitting fluid flow. Such isolators provide a high electrical resistance path that limits electrical current flow between two fitting connections, but allows for the gradual dissipation of electrostatic charge. However, investigation of alternative methods continues to provide additional protection against sparking and damage resulting from lightning strikes and other electromagnetic effects.
Accordingly, those skilled in the art continue to seek new techniques for avoiding damage resulting from lightning strikes.